1. Field of the Invention
The present invention relates generally to optical systems, and more particularly to adaptive optics systems designed to compensate for phase aberrations in a light beam.
2. Description of the Related Art
An "adaptive optics" system is an optical system having components whose characteristics are controlled in real time, that is, during actual operation, to correct for wavefront aberrations. An important application for adaptive optics is the reduction of phase aberrations induced in a light beam by the atmosphere. For example, atmospheric turbulence can severely aberrate a laser beam being transmitted through the atmosphere to a target receiver or detector. An adaptive optics system can be used to compensate for such aberrations so that the beam is free of aberrations when it arrives at the receiving site. Deformable mirrors have been used to accomplish this compensation.
In a deformable mirror adaptive optics system, a reference beam reflected from a target propagates through the atmosphere to the deformable mirror. The reference beam is aberrated as it travels through the atmosphere. The aberrated reference beam is reflected by the mirror and a portion of the reflected beam is applied to a mirror control means such as a wavefront sensor. The wavefront sensor generates a set of optical wavefront error signals. These error signals are applied to a series of actuators to physically deform the deformable mirror to compensate for the aberrations in the reference beam.
More particularly, the mirror is deformed so as to induce in the received reference beam aberrations which are a conjugate, or in simplified terms an inverted replica, of the aberrations induced by the atmosphere. The effects of the atmospheric aberrations are canceled by the effects of these conjugate aberrations during the reflection process. Consequently, the reflected beam emerges from the mirror virtually aberration-free. Another class of adaptive optics mirror systems uses individually-addressed electro-optic phase shifters instead of deformable mirror mechanical actuators and has similar operational characteristics.
In some applications, after the mirror has been deformed in the manner described above, a second laser beam is directed toward the mirror. This second beam is reflected by the mirror and thence transmitted through the atmosphere toward the target. During this reflection process, the deformed mirror aberrates the second beam by inducing therein aberrations which are the conjugate of the aberrations that were induced in the reference beam by the atmosphere. As the second beam travels from the mirror to the target, it backtracks along the path of the reference beam and is aberrated by the same atmospheric turbulence that aberrated the reference beam. However, the second beam, having been pre-aberrated by the mirror, begins its journey carrying an aberration pattern which is the exact conjugate of the atmosphere-induced aberration pattern. Accordingly, as the second beam propagates through the atmosphere, the aberrations induced by the atmosphere cancel the aberrations induced by the mirror, and the beam arrives at the target substantially free of aberrations.
In other applications the `reference` beam comprises a communications beam which carries information. The operation is essentially identical to that described above. After the aberrations in the communications beam are removed, the beam can be focused onto a coherent detector at the receiving site, with greatly improved efficiency and data handling capability due to the low phase distortion in the beam.
Atmospheric aberrations are often characterized by a relatively wide dynamic range measured in optical path differences. An adaptive optics system which uses a deformable mirror generally has a wide dynamic range and accordingly is well-suited to compensate for such aberrations. However, atmospheric aberrations may also be characterized by rapid spatial variations over a crosssection of the optical path encompassed by the laser beam, and the deformable mirror system does not have sufficiently fine resolution to compensate for such rapid spatial variations at all points. The reason for this is primarily mechanical since only a limited number of actuators can physically fit behind the mirror, and therefore the resolution of the deformable mirror system is correspondingly limited.
Some of these shortcomings of a deformable mirror system can be overcome by utilizing a liquid crystal light valve ("LCLV") system instead of a deformable mirror and the associated electro-mechanical control system. A typical LCLV for this application is a layered device comprising a mirror sandwiched between a liquid crystal layer and a photoconducting layer. This sandwich assembly is positioned between a pair of electrically-conductive elements. An electric potential applied across these elements generates an electric field that extends perpendicularly through the sandwich assembly. The conductivity of the photoconducting layer can be altered by projecting onto its back side a pattern of light of spatially-varying intensity. Such a change in conductivity spatially perturbs the electric field, and the perturbed electric field in turn causes the liquid crystal layer to induce, in a light beam propagating therethrough, a pattern of phase aberrations corresponding with the light intensity pattern being projected onto the back side of the photoconducting layer.
In an LCLV adaptive optics system, an aberrated reference beam from the target is directed through the liquid crystal layer toward the mirror and is reflected back through the liquid crystal layer by the mirror. After the reflected reference beam emerges from the liquid crystal layer, a portion of the beam is guided around to the other side of the LCLV toward the back side of the photoconducting layer. A local reference light beam, typically a monochromatic plane wave which may be provided by a local reference source, is combined with the reflected reference beam to create an interference pattern wherein the intensity variations are indicative of the phase aberrations in the reflected reference beam. This interference pattern is projected onto the photoconducting layer.
The photoconducting layer spatially perturbs the electric field according to the interference pattern, and the perturbed electric field in turn causes the liquid crystal layer to induce in the reference beam aberrations which are the conjugate of the aberrations induced in the beam by the atmosphere represented by the interference pattern. The effects of the atmosphere-induced aberrations are canceled by the effects of the conjugate aberrations and consequently the reflected reference beam emerges from the liquid crystal layer virtually free of aberrations.
A laser beam that is to be transmitted to the target is then projected toward the liquid crystal (LC) layer. This second beam passes through the LC layer into the LCLV and then is reflected by the mirror back out of the LCLV and thence toward the target. During this process, the LC layer induces in the second beam aberrations which are the same as the aberrations induced by the LC layer in the reference beam, and these aberrations are the conjugate of the aberrations that were originally induced in the reference beam by the atmosphere. Then, as the second beam travels through the atmosphere toward the target, aberrations are induced by the atmosphere. These atmospheric aberrations cancel the aberrations induced in the second beam by the LC layer, and the second beam arrives at the target virtually aberration-free.
Although the spatial resolution of the LCLV is much finer than that of the deformable mirror system, the LCLV has a much narrower dynamic range, and, like the deformable mirror system, it can only compensate for some classes of the aberrations induced by the atmosphere.
It will be apparent from the foregoing that in many instances neither a deformable mirror adaptive optics system nor an LCLV adaptive optics system can fully compensate for atmosphere-induced aberrations in a laser beam. Accordingly, there is a need for an adaptive optics system having both the large dynamic range characteristic of a deformable mirror system and the fine resolution characteristic of an LCLV system.